50 THE MICROSCOPE. 



them the form best adapted for distinct vision by the 

 eye-glass e e. The field-glass has at the same time 

 brought the blue and red images closer together, so 

 that they are adapted to pass uncoloured through the 

 eye-glass. To render this important point more intel- 

 ligible, let it be supposed that the object-glass had 

 not been over-corrected, that it had been perfectly 

 achromatic ; the rays would then have become coloured 

 as soon as they had passed the field-glass; the blue 

 rays, to take the central pencil, for example, would 

 converge at &", and the red rays at /', which is just 

 the reverse of what the eye-lens requires ; for as its- 

 blue focus is also shorter than its red, it would demand 

 rather that the blue image should be at r", and the 

 red at W. This effect has already been referred to as 

 due to over- correction of the object-glass, which re- 

 moves the blue foci b 6 as much beyond the red foci 

 r r as the sum of the distances between the red and 

 the blue foci of the field-lens and eye-lens ; so that 

 the separation b r is exactly taken up in passing 

 through those two lenses, and the several colours 

 coincide, so far as focal distance is concerned, as the 

 rays pass the eye-lens. But while they coincide as to 

 distance, they differ in another respect, the blue 

 images are rendered smaller than the red by the 

 greater refractive power of the field-glass upon the 

 blue rays. In tracing the pencil Z, for instance, it 

 will be noticed that, after passing the field-glass, two 

 sets of lines are drawn, one whole and one dotted, the 

 former representing the red, and the latter the blue 

 rays. This is the accidental effect in the Huyghenian 

 eye-piece pointed out by Boscovich. The separation 

 into colours of the field-glass is like the over-cor- 

 rection of the object-glass, and leads to subsequent 

 complete correction. For if the differently coloured 

 rays were kept together till they reached the eye-glass, 

 they would then become coloured, and present coloured 

 images to the eye; but fortunately, and most use- 

 fully, the separation effected by the field-glass causes 

 the blue rays to fall so much nearer the centre of the 

 eye-glass, where, owing to the spherical figure, the 



